1. Field of the Invention
This invention relates to an image transfer conveying device provided in an image forming apparatus applying an electrophotographic method, such as a copying machine, printer, facsimile, etc., and more particularly, to an image transfer conveying device implementing an electrostatic transfer utilizing a transfer belt and which removes residual toner from the transfer belt.
2. Discussion of the Background
In an image forming apparatus utilizing an electrophotographic method such as a copying machine, printer, facsimile, etc., after a photoconductive element is charged, an image from an original document is exposed or optically written on the photoconductive element, according to image information, with a laser scanning optical system or LED optical writing system, to thereby form an electrostatic latent image on the photoconductive element. The latent image is then developed with toner of a developer, and thereby a visible toner image is realized. This toner image is ultimately transferred to an image transfer member such as a paper toner sheet, etc. conveyed from a paper supplying section. The image transfer member with the image transferred thereto is then conveyed to a fixing device, and thus the toner image is fixed on the image transfer member by the fixing device to obtain a copy or print. In such an image forming apparatus, the image information formed on the photoconductive element with toner can also be intermediately transferred onto an image transfer belt prior to being transferred to the image transfer member.
FIG. 8 is a perspective view showing an example of a basic structure of a background image transfer conveying device 1. FIG. 9 is an explanatory view showing a main construction of the image forming apparatus having the image transfer conveying device as shown in FIG. 8. The image transfer conveying device 1 in FIG. 8 supports a detachable belt unit 2 on a main body 1A. The belt unit 2 includes an image transfer belt 6 movably positioned around a pair of rollers 4 and 5 for receiving a developed image from a photoconductive element 3 having a drum-shape. A DC solenoid 8 connects (contacts) and disconnects (separates) the image transfer belt 6 to and from the photoconductive element 3 via a connecting and disconnecting lever 9. An image transfer bias roller 11 applies an image transfer bias to the image transfer belt 6, and a contacting plate 13 discharges the image transfer belt 6 as illustrated in FIG. 9.
Also, a cleaning device 16 having a cleaning blade 16A removes residual toner and wipes off paper powder (from an image transfer sheet S, see FIG. 10) which adheres to a surface of the image transfer belt 6. A high voltage power source 12 applies a voltage to the image transfer bias roller 11 and is provided in the main body 1A in FIG. 8. As also shown in FIG. 8, a drive roller 5 has a gear 5b connected to a drive motor (not shown), and drive roller 5 is rotation driven. The image transfer belt 6 can be moved in a direction of conveying the image transfer sheet S (in a direction indicated by an arrow A in FIG. 9) at a position facing the photoconductive element 3, following the rotating direction of the drive roller 5.
As shown in FIG. 11, the image transfer belt 6 has a two-layered 6a, 6b structure, and as for its electric resistance measured according to JISK 6911 at the time of applying DC 100 V, the surface resistivity on the belt surface of the surface layer 6b can be set to be 1.times.10.sup.9 .about.1.times.10.sup.12, the surface resistivity rate of an inside layer 6a can be set to be 1.times.10.sup.7 .about.1.times.10.sup.9, and the volume resistivity can be set to be 5.times.10.sup.8 cm.about.5.times.10.sup.10 cm.
As shown in FIGS. 8 and 9, the pair of rollers 4 and 5 are supported by a supporter 7 to rotate freely. The supporter 7 can move by making a fulcrum of the supporting axis 5a of the roller 5 positioned downstream of an image transfer position against the photoconductive element 3 in an advancing direction of image transfer paper S indicated with the arrow A, out of rollers 4 and 5. This supporter 7 is operated by the DC solenoid 8 which is driven by a signal from a control panel 8A on an image transfer position side. That is, the DC solenoid 8 is linked with the connecting and disconnecting lever 9, which makes the supporter 7 move, enabling the image transfer belt 6 to contact or separate from the photoconductive element 3. FIG. 9 shows a state of the image transfer belt 6 separated from the photoconductive element 3, and FIG. 10 shows a state of the image transfer belt 6 contacting the photoconductive element 3.
When a tip end of the image transfer sheet S is aligned with a leading edge of the image formed on the photoconductive element 3 by the registration roller pair 10, the control panel 8A drives the DC solenoid 8 with a drive signal. Accordingly, as shown in FIG. 10, by driving this DC solenoid 8, the supporter 7 approaches the photoconductive element 3, and by contacting the image transfer belt 6 to the photoconductive element 3, a nip section B is formed so that the image transfer sheet S contacts the photoconductive element 3 at a position opposite the photoconductive element 3. The roller 4 operates as a follower roller to follow drive roller 5, and the surface of roller 4 can be in a shape of a taper with both sides of the drive roller 5 in an axis direction having sharp tips, which prevents deviation of image transfer belt 6. The roller 4 is a conductive roller made of conductive metal, etc., but only supports the image transfer belt 6 having an electric-resistance as mentioned above, and is not directly connected to any other conductive members electrically.
Also, the materials for the drive roller 5 can be selected from EPD rubber, chloroprene rubber, or silicone rubber to enhance the power of gripping the image transfer belt 6 at the time of driving.
The image transfer bias roller 11 is connected to an inside of the image transfer belt 6 downstream of the following roller 4 in a moving direction of the image transfer belt 6. This image transfer bias roller 11 constitutes a connecting electrode for providing an electric charge with a polarity inverse to a polarity of the toner on the photoconductive element 3 against the image transfer belt 6, and is connected to the high voltage power source 12.
The contacting plate 13 can be provided on an inside face of the image transfer belt 6 near the following roller 4 at a side below where the image transfer sheet S contacts the image transfer belt 6, contacting plate 13 prevents a charge on the image transfer sheet S upstream of the image transfer nip section B. Also, this contacting plate 13 can detect current flowing on the image transfer belt 6 as a feedback current, and this current detection can control the current supplied from the image transfer bias roller 11. Therefore, the contacting plate 13 is connected to an image transfer control panel 14 to set up the current to be supplied to the image transfer bias roller 11 according to the detected current, and this image transfer control panel 14 is connected to the high voltage power source 12. A location at which the contacting plate 13 is provided is not limited to the inner face of the image transfer belt 6 near the following roller 4, and the contacting plate 13 may also be provided at an inner face of the image transfer belt 6 near the drive roller 5 or at other positions.
As shown in FIG. 10, for such an image transfer conveying device 1, the states and positions of the parts are set up so that the supporter 7 moves the image transfer belt 6 to approach the photoconductive element 3, in conformance to the image transfer sheet S being sent out from registration rollers pair 10. Further, the nip section B, which may have a width of 4 mm to 8 mm corresponding to a length along the conveying direction of the image transfer sheet S, is formed between the image transfer belt 6 and the photoconductive element 3.
FIG. 12 shows image transfer in the device of FIGS. 8-11. When the surface of photoconductive element 3 is in a state of charging at, for example, -800 V as shown in FIG. 12, positively charged toner electrostatically adhered on the surface of the photoconductive element 3 is moved to the nip section B (see FIG. 10). At the time that a part of the photoconductive element 3 is positioned near the image transfer belt 6 before coming to the nip section B, the surface potential of the photoconductive element 3 is lowered by a pre-transfer discharge lamp 15 (PTL 15) positioned adjacent the photoconductive element 3 which weakens the charge on the surface of photoconductive element 3. FIG. 12 shows the degree of charge according to a size of the circle marks, and a state of a toner charge after lowering by PTL 15 is shown as smaller circles compared to the size of the larger circle marks indicating a charge before discharge.
In the nip section B, the toner on the photoconductive element 3 is transferred to the image transfer sheet S by the image transfer bias from the image transfer bias roller 11 positioned (or located) at an opposite side (or inside) of the image transfer belt 6. This image transfer bias is applied from the high voltage power source 12 in the range of -1.5 k V to 6.5 k V, but as a result of the below-mentioned constant-current control operation, the image transfer bias is variably set up. That is, in FIGS. 9 and 10, when the current value output from the high voltage power source 12 is I and a value in a case of detecting a value of feedback current flowing from the contacting plate 13 via the image transfer belt 6 is 12, the value of II is controlled so that the following relation can be acquired between both the values: EQU I1-I2=I OUT(but, I OUT: constant) (1)
This eliminates variation in the efficiency of image transfer by stabilizing the surface potential Vp on the image transfer sheet S regardless of changes in temperature, humidity, other surrounding conditions, unevenness arising from a manufactured quality of image transfer belt 6, etc.
In other words, by deeming the above as the current I OUT flowing to the photoconductive element 3 via the image transfer belt 6 and the image transfer sheet S, the flowing ability of current to the image transfer belt 6 can vary in consequence of, e.g., resistance lowering or heightening of a surface resistance Vp on the image transfer sheet S, to thereby prevent any adverse influence on the separating ability and image transferring performance of the image transfer sheet S.
In this example, in a case of a conveying velocity of 330 mm/sec, an effective image transfer bias roller length of 310 mm, I OUT=35 A-5 A, such that I OUT can obtain a good image transfer.
Further, image transfer is conducted from the photoconductive element 3, and the image transfer sheet S is also charged simultaneously. Accordingly, due to a relation of a true electric charge of the image transfer belt 6 to a polarization charge generated at image transfer sheet S, the image transfer sheet S is electrostatically attracted to the image transfer belt 6, to enable the image transfer sheet S to separate from the photoconductive element 3. Also, a peeling-off operation by a stiffness of the image transfer sheet S utilizing the curvature separation of the photoconductive element 3 enables the above separation.
However, such an electrostatic attraction gives rise to a possibility that the image transfer sheet S will not be separated smoothly from the image transfer belt 6, depending on a variation of surrounding conditions, particularly in a case of high humidity which causes current to flow to the image transfer sheet S. Therefore, setting up a somewhat higher resistance value on the surface layer 6b of the image transfer belt 6 allows a delayed transition of a true charge on the image transfer sheet S in the nip section B. Thereby, a transition of the true electric charge from the image transfer belt 6 to the image transfer sheet S can be delayed, which prevents electrostatic attraction relations between the image transfer sheet S and photoconductive element 3.
The delayed transition of true electric charge in this case means that no charge is generated on the image transfer sheet S upstream to until the image transfer sheet S extends to the nip section B on the side of photoconductive element 3. Thereby, winding of the image transfer sheet S into the photoconductive element 3 is prevented, and also faulty separating of the image transfer sheet S from the photoconductive element 3 is prevented.
Further, materials with an invariable resistance depending on a change of surrounding conditions may be favorably selected as materials for the image transfer belt 6. As conductive materials to control resistance, when adding a proper volume of carbon, zinc oxide and the like, and when using a rubber belt as an elastic belt, materials with less hygoscopicity and stabilized resistance value such as chloroprene rubber, EPD rubber, silicone rubber, epichlorohydrin rubber may be desirably selected.
Moreover, the value of current I OUT flowing to the photoconductive element 3 can be reduced in a case of a low conveying speed, and inversely such a value can be increased when the conveying speed is high or the PTL 15 is not used.
Whereas the image transfer sheet S passing the nip section B is electrostatically attracted to and carried in conformity to the movement of the image transfer belt 6, curvature separation occurs from the drive roller 5. Thereby, the diameter of drive roller 5 can be set not to exceed 16 mm. Further, when using such a drive roller 5, experiments have shown that to achieve such a separating effect high quality 45 K paper (rigidity: horizontal 21 cm.sup.3 /100!) can preferably be utilized.
Also, as shown in FIG. 9, the image transfer sheet S separated from the image transfer belt 6 with the drive roller 5 is then carried between a heating roller 17a included in fixing section 17, by being guided by a guide plate, and a pad roller 17b. The fixing section 17 fixes the toner image onto the image transfer sheet S by heating and melting the toner on the image transfer sheet S and by contacting and pressing the toner image onto the image transfer sheet S.
When the image transfer belt 6 completes the image transfer and is separated from the image transfer sheet S, supporter 7 supporting the image transfer belt 6 is separated from the photoconductive element 3, by releasing the connecting and disconnecting lever 9 according to a releasing excitation of DC solenoid 8. The surface of the image transfer belt 6 is then cleaned by cleaning device 16. The cleaning device 16 includes cleaning blade 16A, which peels off toner or paper powder of the image transfer sheet S from the image transfer belt 6.
The image transfer belt 6 is frictionally contacted by the cleaning blade 16A. The image transfer belt 6 may be covered by fluorine contained resin on its surface to lower a friction coefficient, or for example, polyvinylidene fluoride, tetrafluoroethylene and the like, to prevent enlarging a required driving power due to any increased sliding and friction resistance. Also, the toner or paper powder removed from the surface of the image transfer belt 6 can be stored into a waste toner recovery container (not shown) from the main body 1 by the toner recovery coil 16B.
In this structure of the image transfer conveying device using the image transfer belt 6, the device for cleaning the image transfer belt may include the cleaning blade 16A only (as in FIGS. 8-10), and a width of a cleaning area of cleaning blade 16A is about 10 mm broader than both ends of an effective image area on the image transfer belt 6.
However, one drawback in the above-discussed background device is that toner on the image transfer belt 6 actually exists not only just outside the effective image forming area, but also to the edges of the image transfer belt 6 due to splashing of toner, i.e. toner splashes onto the image transfer belt 6 outside of 10 mm beyond the effective image forming area. Also, when cleaning the toner on the edges of the image transfer belt 6 by using the cleaning blade 16A, the edges of the cleaning blade 16A may entangle at the edge sections. Therefore, the length of the cleaning blade 16A is made shorter than the width of the image transfer belt 6, and toner at the edge sections of the image transfer belt 6 can not be cleaned by the cleaning blade 16A, but may remain on the edges of the image transfer belt 6. When toner is attached to edge sections of the image transfer belt 6 on a front side of the image forming device, i.e. a side to which a door of the image forming device may open in servicing the image forming device, for example to address a paperjamming, there may arise a problem of an operator's hand getting dirty from such remaining toner in a case of the operator inserting a hand into the belt section for treatment of the paper jamming and the like.
Also with the device as shown in FIGS. 8-11, paper powder of a transfer sheet may adhere onto a tip end edge of the cleaning blade 16A, and the cleaning blade 16A may then be in a state of separating from and not properly contacting the image transfer belt 6. As a result, residual toner slips down through a gap between the cleaning blade 16A and the image transfer belt 6, allowing dirt to fall on a back face of a transfer paper sheet S or to slip down past the cleaning blade 16A to fix onto the image transfer belt 6.
To solve such a problem, an applicant of this patent application has proposed a construction of a cleaning device as illustrated in FIG. 13.
FIG. 13 shows the image transfer belt 6, the drive roller 5, and a cleaning device 24 on the whole, having cleaning blade 16A as a main cleaning device for the cleaning device 24, and cleaning member 28 as a subsidiary cleaning device for the cleaning device 24. The cleaning member 28 includes a bias roller 28A, which applies an electric field with an inverse polarity to a polarity of an applied electric field of the bias roller 11 providing the electric charge to the image transfer belt 6, and a blade 28b to remove toner adhering to the surface of the bias roller 28a. Further, the bias roller 28a rotates in a reverse direction to the advancing direction of the image transfer belt 6.
Such a cleaning device has advantages in a case that paper powder and the like of a transfer paper sheet S adheres to a tip portion of an edge of cleaning blade 16A and residual toner slips down past the cleaning blade 16A, specifically that the slipping toner will adhere to the bias roller 28a rotating in the reverse direction against the advancing direction of the image transfer belt 6 downstream of rotation of the image transfer belt 6, which enables residual toner to be wiped off and which prevents dirt from reaching a back face of the transfer sheet S and an end face of an edge of the cleaning blade 16A.
However, this structure still does not adequately remove toner from edges of image transfer belt 6. Further the bias roller 28a and the image transfer belt 6 are attracted to each other by an electrostatic force, since the cleaning device applies a bias voltage by the bias roller 28a. Further, the bias roller 28a rotates in opposition to the rotation direction of the image transfer belt 6. Therefore, a problem occurs that a noise arises by the rubbing together of the bias roller 28a and the image transfer belt 6, which noise particularly arises from bias roller 28a rotating in an opposite direction to image transfer belt 6. Further, the surface coating of image transfer belt 6 is excessively scraped, and as a result cleaning efficiency decreases, and scraping the surface coating causes a friction coefficient on the image transfer belt 6 surface to increase, which results in entangling make global change of the cleaning blade 16A as a main cleaning device, when the bias roller 28a rotates in the reverse direction against the advancing direction of the image transfer belt 6. Further, as the bias roller 28a rotates against the advancing direction of the image transfer belt 6, the image transfer belt 6 may slacken around its contact point with the bias roller 28a. These drawbacks are particularly significant if the image transfer belt 6 is made of material like rubber, etc., as opposed to being made of a slippery resin.